Aluminum is conventionally manufactured by the electrolysis of alumina (Al.sub.2 O.sub.3) which is reduced to metallic aluminum. The reduction is done in a bath of fused cryolite (3NaF.AlF.sub.3). This reduction reaction is done in a pot with a carbonaceous potlining for the cryolite bath, the pot serving as the cathode in an electrolysis reaction at about 1600.degree. F. to about 1800.degree. F. During use over extended periods of time, the carbonaceous potlining of the pot gradually deteriorates, the carbonaceous lining material being penetrated by the materials in the pot which causes leakage of bath and aluminum metals.
Hundreds of thousands of tons of spent potlinings are generated each year in the production of aluminum. Spent carbonaceous potlinings, as a result of their use, deterioration and penetration by bath materials, contain significant quantities of fluorides as well as aluminum, alkali metals, nitrides, and some cyanide. Because spent potlinings comprise various materials which include such hazardous wastes such as fluorides and cyanide, the generation, storage and disposal of the potlinings have an environmental impact. Because a significant portion of spent potlinings is carbonaceous, the burning or the combustion of these spent potlinings is a logical way of meeting disposal problems. Indeed there are a number of such patents which are directed to this general concept, however, most of them such as U.S. Pat. No. 4,444,740 to Snodgrass et al. and U.S. Pat. No. 4,053,375 to Roberts et al. are concerned with either the alleviation of a potential environmental problem with the output of the combustor or with the recovery of some reusable material such as cryolite or fluorine from the potlinings.
Combustion of spent potlinings provides a highly efficient means for destroying the carbonaceous potlining, and cyanide therein as well as allows the fluoride salts to be more conveniently treated either in the combustor or in post-combustion processes. For example in connection with cyanide, at temperatures of about 1,400.degree. F. cyanide is oxidized to N.sub.2, CO.sub.2 and nitrogen oxides. Stable combustion of spent potlinings has been found to require temperatures which exceed 1400.degree. F. At these temperatures many potlining particles and ash therefrom exhibit agglomeration tendencies. Agglomeration results from salts such as alkali metal salts, halogen salts and silicon salts reacting and then melting at combustion temperatures. These reacting and melting salts create the conditions for the agglomeration of potlining particulates. Agglomeration of these particles inhibits their complete combustion. Moreover, agglomeration is deleterious to combustors and interferes with the ability of most combustors to continue to contact the potlining particles with air to continue stable combustion.
In fluidized-bed combustors and circulating bed combustors, agglomeration adversely affects the beds and their ability to contact potlinings with air for complete stable combustion. Indeed a report done for the United States Environmental Protection Agency by Battelle Columbus Laboratories (Report No. EPA-600/2-84-064, hereinafter "the Report") addresses the problem of agglomeration of potlinings in fluidized-bed combustors. The Report recognized agglomeration of potlining particles during combustion as an unsolved problem which the Report suggested could be addressed only by reducing the temperature of the combustor to below 1400.degree. F. and accepting incomplete but "substantial" burning of the potlining. Moreover, while the Report discussed the use of kaolin clay with the ash from the combustion of spent potlinings, the Report did not suggest the use of kaolin clay or particulate additives with comminuted potlinings, as opposed to potlining ash, to completely combust spent potlinings at 1,400.degree. F. or greater.
An object of the invention is to provide for the complete combustion of spent carbonaceous potlinings which are generated during the production of aluminum.
Another object of the invention is to prevent the agglomeration of comminuted carbonaceous potlinings during their combustion.
Another object of the invention is to provide a safe and ecologically sound process for disposing of spent potlinings without emitting deleterious materials into the environment.
Yet another object of the invention is to provide for the combustion of potlinings as a fuel source for the generation of thermal energy.